The present invention is related to a method for controlling a rolling mill comprising at least two mill stands arranged after each other, wherein a first stand is arranged upstream of a second stand, each of said stands comprising two spaced rolls, wherein an elongated material is fed between the rolls of each stand by rotating the rolls, the width of the material being measured at a location at least downstream of said first stand. More particularly, it relates to a rolling mill for the production of materials with shapes different from sheets or strips, such as rods and bars of various types.
The present invention is further related to a device for controlling a rolling mill.
A rolling mill normally comprises a plurality of mill stands arranged after each other. Each of said stands comprises two spaced rolls with parallel rotation axes. A material is fed between the rolls of each stand, and thereby rolled, by rotating the rolls. The rolled material will elongate and spread as the cross section of the rolled material is reduced as it passes through said stands. The cross section after each stand is defined by the passdesign and the layout of the mill. The cross section is defined by the height and the width of the material leaving a roll gap.
Typically, the rolls of a first stand rolls the material in a first direction, and the rolls of a second, adjacent stand rolls the material in a direction perpendicular to the first direction. Usually, the rolls of said first stand have horizontal rotation axes, and the rolls of said second stand have vertically directed rotation axes. Thus, a vertical dimension of the rolled material is reduced in said first stand and a horizontal dimension of the rolled material is reduced in said second stand.
Each of the rolls of each stand has a groove shaped for the specific, desired profile according to the passdesign for the rolled material. The depth of the grooves and the gap between the rolls determines the height of the rolled material in a direction perpendicular to the rotation axes of the rolls. The grooves of the rolls are worn during rolling and the gap between the rolls of a stand is therefore needed to be decreased.
The width of the material is defined in a direction parallel to the axes of the rolls. The width is affected by the spread that varies with temperature, material and the interstand tension between two stands. The spread is not constant from head to tail of a rolled billet.
To keep the different stands cross section constant and equal to the passdesign during rolling, the height must be compensated for roll wear and the width must be compensated for the spread from head to tail of a billet.
A rolling mill is typically divided into three sections, called a roughing, an intermediate and a finishing section. The material enters the roughing section first and is reduced in cross section during its passage of said sections.
The materials enter the roughing section as billets which have been heated to a previously determined temperature of, for example, around 1 000xc2x0 C. for a typical steel. The billets may for example enter the first stand of the roughing section with a speed of around 0.5 m/s and dimensions of, for example, 140 mmxc3x97140 mm. As the material proceeds downstream through the rolling mill, it is reduced in cross section and accelerated in speed. The materials may for example exit the final stand of the finishing section at a speed of around 120 m/s with a diameter of, for example, 5.5 mm.
Today, primarily the cross section of the rolled material is controlled by the shape of the grooves and the distance between the rolls, roll gap, of each stand. In order to, in this way, achieve the desired cross sectional shape of the material downstream of a second mill stand, it is necessary that the back tension between said second stand and an upstream adjacent first stand, and the front tension between said second stand and an adjacent downstream stand is approximately zero.
In GB 2 009 974, a method of controlling a rolling mill is described. The width of a material is measured downstream of a second stand. Errors in the width are detected and the rotational speed of the rolls of said second stand is adjusted in a sense to minimise the detected errors in the width. The interstand tension between a first stand, arranged upstream of said second stand, and said second stand is measured by means of transducers, The roll separation of said first stand is thereafter adjusted in a sense to minimise the interstand tension. The purpose of the adjustments is to minimise the interstand tension and regulate the width by means of the roll separation and thus the mass flow. A disadvantage of this method is that the mechanical design of the stand must allow the roll gap to be operated under load, i.e. with material in the roll gap. Most mill stands of old design as well as modern cantilever stands will not meet this requirement. Another disadvantage is that transducers are needed for measuring and controlling the tension.
The object of the invention is to devise ways to achieve an accurate control of the dimensions of a rolled material. A further object of the invention is to provide a method for controlling the dimensions of the rolled material, that is applicable in any section of a rolling mill.
These objects are achieved in that, if the measured width is not within predefined first upper and lower limit values, an interstand tension between said first and second stand is adjusted to a value corresponding to the deviation of the measured width from said predefined limit values in order to control the width of said material to be within said first upper and lower limit values. Thus, different interstand tension values are required for compensating different widths. The interstand tension is further allowed to vary along the length of each material. Control of the interstand tension is used for regulating the width. In this way, no further action is needed for regulating the width. This implies that the rolls of each of the stands do not need to be operated against load. The production of a rolling mill arranged for being controlled with this method is therefore cost-effective.
The width of a first portion of the material is measured at said location. The width of a second portion of the material, said second portion being located behind said first portion in the feeding direction is controlled to be within said limit values.
According to a preferred embodiment of the invention, the width of the material is measured at a location downstream of said second stand. In this way, the interstand tension between the first and second stand is effectively adjusted by means of back tension.
According to another preferred embodiment of the invention, if the measured width exceeds the upper limit value, the rotational speed of the rolls of said first stand is decreased in relation to the rotational speed of the rolls of said second stand, and, if the measured width is below the lower limit value, the rotational speed of the rolls of said first stand is increased in relation to the rotational speed of the rolls of said second stand. By changing the rotational speed of the rolls of said first stand in relation to the rotational speed of rolls of said second stand, an accurate and easy control of the interstand tension is achieved.
According another embodiment of the invention, said measuring is performed a plurality of times for each material. Preferably, the measuring of said width and the corresponding adjustment of the interstand tension is performed with very small time intervals such as less than 0.05 seconds. In this way a very accurate control of the dimensions of the rolled material is achieved.
By substantially continues measuring the cross section key dimensions out of a second stand and controlling the width by changing the roll speed of a first stand allowing the back tension as well as front tension to change during rolling, there is no need for tension transducers or mill stands that can be operated against load. The roll gap is only controlled in between billets to compensate for roll wear. This method is simple, reliable and is applicable to all types of mill stands.
According to another embodiment of the invention, a position of the material is detected at a location between said first and second stand when the material is in contact with both said first and second stand, and that, if the detected position is not within defined position limit values, a present compressive stress in the material between said first and second stand is. According to the inventive method, the material is preferably fed along a substantially straight line between said stands. Due to that the cross sectional dimensions of the roiled material is controlled by adjusting the interstand tension, a compressive force in the material may arise between said first and second stand. An uncontrolled compression force in the roiled material may however lead to a cobble, which is an out-of-control situation during which the rolled material suddenly shoots out of the normal pathway between two mill stands and may be thrown all over the mill. Due to the last mentioned embodiment, a difference in position for the rolled material in relation to a desired pathway indicates an undesired bend of the material resulting from the compressive force in the material being too large. Decreasing the compressive stress in the material, the rolled material will be straightened out and a cobble is avoided.
The inventive device for controlling a rolling mill is more closely defined in the claims and the following description.